Physics 21
Practice Quiz#11
Name _____________________
b 1. The observed relativistic length of a super rocket moving by the observer at 0.70 c will be what factor times that
of the measured rocket length if it were at rest?
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a. 0.45
b. 0.71
c. 0.82
d. 1.4
b 2. A quantum of radiation has an energy of 2.0 keV. What is its frequency? (h = 6.63  1034 Js and 1 eV = 1.60 
1019 J)
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a. 3.2  1017 Hz
b. 4.8  1017 Hz
c. 6.3  1017 Hz
d. 7.3  1017 Hz
d 3.
The ionization energy of the hydrogen atom is 13.6 eV. What is the energy of the n = 5 state?
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a. 2.72 eV
b. –2.72 eV
c. 0.544 eV
d. –0.544 eV
b 4. A radioactive material initially is observed to have an activity of 1 000 decays/sec. If three hours later it is
observed to have an activity of 125 decays/sec, what is its half-life?
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a. 1/2 hour
b. 1 hour
c. 3 hours
d. 8 hours